Commas (,) are always used in pairs. Sometimes one of the commas is subsumed into a full stop or is implied at the start of the sentence (as with dependent clauses, which start with an implied comma and finish with a visible comma). They are used to give additional information about a topic. The sentence would make sense without the information within the commas.
e.g. The tube, with a 3mm diameter, is inserted into the entrance point.
Note: it is important not to separate the subject from the verb with a single comma: commas should always be used in pairs to show discrete information. Parentheses or brackets (...) are used to separate off complementary information. The separation is slightly longer and more definite than that for commas.
e.g. The probe (Ernst and Braun, Berlin) is essential.
Semi colons (;) are used to show two, developing independent clauses. They are used to show that the second clause is a development of the first idea. They can also be used in place of a connective to show the relationship between the two clauses.
e.g. The probe is used for measurement; a cobra probe is ideal for this purpose.
Semi colons can also be used to separate out long lists of items with internal commas.
e.g. The probe is used for measurement, both formal and informal; testing of heat generation; and testing of particle flow and other events.
Colons (:) are used to link two, directly-linked independent clauses. They can be used to replace a connective. They are used to expand the key idea.
e.g. The probe is used to measure flow: the flow rate is increased every 30 seconds.
Colons (:) can sometimes be used to introduce a long list; however this is becoming increasingly uncommon in academic writing.
e.g. The probe has three uses: measuring, testing and data gathering.
Full stops (.) are used to show the end of a sentence.
e.g. The tube is inserted.
Full stops are also used to show an abbreviation where the last letter of the original word is not given.
e.g. Prof. (Professor) but not Dr (Doctor).
Exclamation marks (!) are used to point out the extreme nature of an idea. They are rarely used in formal academic writing.
e.g. The explosion following the mixing of the chemicals was unexpected!
Question marks (?) are used to show the end of a question.
e.g. Should these chemicals be mixed?
Quotation marks (“...”) are used to show that a word or words is/are not your own.
e.g. Brevin (1987) argues, “these chemicals should not be mixed”.
Note: direct quotation of this kind is rarely seen in academic engineering writing.
Apostrophes (’) are used in two cases: to show possession and to show omission.
Singular possession: Alison’s book. This can be checked by turning the sentence around: the book of Alison.
Plural possession: The cats’ home. Again, this can be turned around to check for possession. As the word cat is being used in the plural form cats, the apostrophe goes after the ‘s’ of cats.
Complication: James’ book. Here the apostrophe goes after James without a second ‘s’, as that would be too challenging to pronounce.
Omission: e.g. don’t, won’t, can’t. Here, the apostrophe replaces the missing letter(s). However, please note that contractions (i.e. shortened forms of words) should not be used in formal academic writing.